Fe C Phase Diagram
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Transcript of Fe C Phase Diagram
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Fe-C Phase Diagram
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At room temperature it exists as ferrite,
or iron.
BCC crystal structure
mostly iron with a little carbon
relatively soft
Upon heating pure Iron experiences two
changes in crystal structure.
When we heat it to 912C it experiences a
polymorphic transformation to austenite,
or iron
FCC crystal structure
Unstable at room temp.
Can accommodate more carbon than
ferrite
At 1394C austenite reverts back to a BCC
phase called ferrite.
1394C
FerriteBCC
912C
ironAustenite
FCC
25C
Ferrite
BCC
1538C
Melts
Pure Iron
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Fe-Fe3C Phase Diagram
Only part of the phase
diagram is shown.
The left axis is pure
iron.
Pure Fe
On the right the phase
diagram only extends
to 6.70 wt%C
Fe3C
cementite
6.70 wt% C
At this concentration
the intermediate
compound iron
carbide, orcementite
(Fe3C) is formed
This is sufficient to
describe all of the
steels and cast irons
used today.
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Austenite
+ Fe3C
Development of Microstructures in Iron-CarbonAlloys
(Pearlite)3CFe+
Various microstructures can be
produced in steel alloys
depending on
carbon content
heat treatment
Equilibrium (slow) cooling from
the region through the eutectoid
composition of 0.76 wt% C
% and Fe3C in eutectoid pearlite
%89100022.07.6
76.07.6=
=
W
%11100022.07.6
022.076.03 =
=CFeW
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Formation of Pearlite
Micrograph of eutectoid
steel, showing pearlite
microstructure.
ferrite (light)
Fe3C (dark)
Schematic representation of
the formation of pearlite
from austenite
direction of arrows
indicates carbon diffusion
0.76 wt% C
0.022 wt%C
6.7 wt%C
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Hypo-eutectoid Composition (wt% C
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Hypo-eutectoid Composition (0.38 wt% C)
White regions:
ProeutectoidFerrite
Dark regions:
Pearlite
Close-spaced
layers
Unresolved at this
magnification
Pearlite
wider-spaced
layers
91 m
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Computing the relative amounts ofproeutectoid and pearlite
Similar to previous lecture.
Use the Lever Rule in
conjunction with a tie line
UT
UW
+='
For hypoeutectic composition
C'0, fractions of pro-eutectic ,
and pearlite are:
C0
hypoeutectic
UT
TWp
+=
(total) and Fe3C
XVUT
XVUW
+++
++=
XVUT
TW CFe
+++=3
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Computing the relative amounts of
proeutectoid and pearlite
XV
VW CFe
+='3
For hypereutectic compositionC'1
C1
hypereutectic
XV
XWp
+=
(total) and Fe3C
XVUT
XW
+++=
XVUT
VUTW CFe
+++
++=3
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Hyper-eutectoid Composition (wt% C >0.76)
Upon cooling enter a two-phase
region
Composition between 0.76 and 2.14
wt% C
The pro-eutectoid cementite phase has
begun to form along the grain
boundaries
Final structure is a mixture of
Pro-eutectoid cementite
Pearlite
CFe3+
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Hyper-eutectoid Composition (1.40 wt% C)
White Pro-
eutectoid cementite
network
Pearlite colonies
30 m
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Mechanical Properties of Steels
The mechanical properties of steel are largely
dictated by the phase transformations they undergo
upon cooling
If we heat steel to the single phase austenite
region and vary the cooling rate we can control the
microstructure.
Understanding phase transformations of metallic
alloys
allows us to design a heat treatment for a specific alloy
that will yield the desired room temperature mechanical
properties
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Example: Railway Rails
Eutectoid composition of 0.76 wt%C
100% Pearlite structure
Pearlite is a natural composite
Hard and Brittle Fe3C plates
Soft and Ductile ferrite plates
The strength of Pearlite is dictated by its
interlaminar spacing, S (m)
14.46140 += Sy
SIncreased cooling rates
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Pearlite
Coarse Pearlite Fine Pearlite
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The rate of transformation
of the austenite to pearlite
is dependent ontemperature
This temperature
dependence can be plotted
as % transformation vs.
log. time
Isothermal Transformation Diagrams
Data was collected for
each curve, after rapid
cooling of 100% austenite
to the temperature
indicated temperature was then
keep constant
throughout the course of
the reaction
Eutectoid composition (0.76 wt% C)
Cool rapidly from 100% austenite
through eutectoid isotherm (727 C) to
600, 650, 675C
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More convenient to
represent time-
temperature dependence
isothermal
transformation diagram
generated from the %
transformation-versus-
log. times measurements
Note the eutectoidtemperature (727C)
Many tests conducted to
these construct curves.
Isothermal Transformation Diagrams
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Isothermal Transformation Diagrams
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Spheroidite
Hold steel at high temperature
Below 727 oC
for a sufficiently long time
the Fe3C (cementite) plates
spheroidize
the continuous phase is ferrite
Large drop in strength occurs
But ductility is greatly
increased
30 m
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Formation of Martensite
Non-Equilibrium Cooling
Carbon must diffuse (move)
in order to form pearlite
low C in -Fe high C in Fe3C
Diffusion takes time +
temperature
If the sample is quenched
(plunged into water)
no time for diffusion
pearlite cannot form
A supersaturated and
unstable structure is formed.
Lattice gets stuckbetween FCC and BCC
i.e. we get Body Centred
Tetragonal (BCT)
structure or in terms of
lattice parameters, ac
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Characteristics of Martensite Transformation
Occurs by a non-diffusional process
Transformation occurs extremely fast, i.e. at the
speed of sound
Transformation occurs at temperatures well below
eutectoid temperature
Martensite is very hard (i.e. high y) but is
completely brittle
Not useful as an engineering material
Must be tempered
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Martensite
Quench Needle shaped
White areas austenite
quenched too fast
Tempered 564C
small particles cementite
matrix is ferrite
24.6 m 2 m
Effect of Carbon Content on Hardness
Brinell Hardness 0 - 320 Brinell Hardness 0 - 750
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Effect of Carbon Content on Yield and
Tensile Strength